Distance is a numerical description of how far apart objects are. In physics or everyday usage, distance may refer to a physical length, or an estimation based on other criteria. In most cases, "distance from A to B" is interchangeable with "distance between B and A".
The distance between (x1, y1) and (x2, y2) is given by:
d=sqrt((x2-x1)² + (y2-y1)²)
Source: Wikipedia (license)